One of the problems encountered when bleaching pulp with oxygen is attempting to wash the pulp after the oxygen stage. The oxygen entrained in the pulp mat reduces the vacuum on the vacuum washer and mixes with the wash water to form foam which reduces the vacuum in the washer and prevents the chemicals and water from passing through the mat.
There have been a number of proposals for the deaerating the pulp before the washer. One proposal is to use a "Deculator" process which is described by Mac Gregor, "The Deaeration of Paper, Paperboard and Hardboard Stock by the Deculator Process," Appita, Vol. 22, No. 6, May 1969, pp. xvii-xxi. It is also described in Paper, Vol. 184, No. 11, 1975, Dec. 8, 1975, pp. 667-668 and in Jacobsson, "Complete Deaeration as a Basic Necessity: The Deculator System," Paper, Apr. 20, 1981, pp. 61-62, 64, and 67. The process involves the rapid acceleration and spraying of stock into a long 6 foot diameter stainless steel receiver where it is boiled under vacuum while being impinged against a suitable target surface. This system involves expensive equipment, including vacuum systems and vacuum vessels.
Another system is described in Sethy, U.S. Pat. No. 4,209,359 issued Jun. 24, 1980. In this system the slurry, usually containing no more that 3% by weight of fiber, is agitated by a radial flow impeller which imparts a substantially radial flow to the slurry. Several types of impeller are shown.
Other large degassing chambers are shown in Roymoulik, et al. U.S. Pat. No. 3,832,276 granted Aug. 27, 1974, and in Richter, U.S. Pat. No. 3,963,561 granted Jun. 15, 1976. These are large expensive systems utilizing large tanks which must be built to withstand pressure. The Richter patent describes a large pressure vessel.